An imaging device usually includes a light-receiving element (photoelectric conversion element or photodiode) formed on a silicon semiconductor substrate. By the way, if a wavelength of incident light is determined, an optical absorption coefficient of silicon (Si) is uniquely determined. Therefore, in order to cause a silicon semiconductor substrate to efficiently absorb light, particularly light in a red to near infrared region, it is necessary to form a light-receiving element in a region of the silicon semiconductor substrate located at a deep position from a light incident surface (specifically, for example, about 10 μm) (refer to, for example, Japanese Patent Application Laid-open No. 09-331058). This means that micronization of a pixel in an imaging device increases an aspect ratio in the light-receiving element.
However, an increase in aspect ratio in a light-receiving element (imaging element) causes such inter-pixel color mixing that incident light on a light-receiving element (referred to as “light-receiving element-B” for convenience) adjacent to a certain light-receiving element (referred to as “light-receiving element-A” for convenience) is incident on the light-receiving element-A disadvantageously. Reducing an aspect ratio in a light-receiving element in order to reduce inter-pixel color mixing causes a decrease in sensitivity of the light-receiving element in a red to near infrared region disadvantageously. In addition, an energy band gap of Si is 1.1 eV, and therefore it is impossible in principle to detect an infrared ray having a longer wavelength than 1.1 μm. An infrared ray can be detected, for example, using a photoelectric conversion layer having a laminated structure of an InP layer and an InGaAs layer in place of Si.